The present invention relates to methods of manufacturing super absorbent composite sheets wherein the making of a web into a non-woven fabric, the bonding of the web to a super absorbent polymer and the bonding of the particles of the super absorbent polymer with each other are all carried out effectively and little super absorbent polymer drops out in both wet and dry states. The present invention also relates to super absorbent composite sheets manufactured by any of such methods.
As an absorbent member for such absorbent article as a baby diaper, an adult incontinence diaper, a feminine hygiene product, a blood absorbent material and a mother""s milk pad, the development of a highly absorbent sheet mainly consisting of a super absorbent resin polymer (SAP) and a wood pulp fluff the SAP being held in a thinner and more dimensionally stable non-woven fabric structure has been made energetically.
In order for a non-woven fabric to hold SAP, such methods as a method where a non-woven fabric having a structure preferable a substrate is prepared, the non-woven fabric is impregnated with acrylic acid monomer so that the monomer is polymerized a method where acrylic acid monomer is polymerized on a non-woven fabric, gelated non-cross-linked polymer is used to coat the non-woven fabric and cross-linking is then performed, a method where slurry of SAP dispersed in a medium is used to coat a non-woven fabric are applied.
First, there may be three fundamental properties required of a non-woven fabric substrate; (1) properties of a supporting member, (2) properties of holding and fixing SAP, and (3) properties to penetrate and disperse. In order for a non-woven fabric to hold SAP in its structure, the non-woven fabric needs to have a bulky structure having spaces among its constituent fibers and, if stated in extreme terms, the bulkier the non-woven fabric, the better the results. If such bulky non-woven fabric is supplied in a bulky wound-up roll from a non-woven fabric manufacturer, however, the transportation would be much costly and the amount of non-woven fabric wound up on roll would be greatly limited.
In such cases, it is conceivable to directly link a step of manufacturing a non-woven fabric to a step of having the non-woven fabric hold SAP, and there is an example in commercial practice of linking a step of manufacturing a thermally bonded non-woven fabric to a step of having the non-woven fabric hold SAP, which may be, however, complicated process-wise and costly initial-investment-wise.
Then, as an alternative a method of having the manufacture of a non-woven fabric and the holding of SAP carried out at the same time in a step of manufacturing a non-woven fabric may be thought of. As an example of such method, SAP in powder form is usually made to co-form with pulp or fiber as SAP is carried on an air stream. But dust may be generated or SAP powder moves inside an absorbent member, which is not desirable. Also, a so-called wet method is patented for; SAP is dispersed in a pulp slurry or a fiber slurry to form a sheet. Such method has a serious inherent limitation in that the fiber concentration is too low and the manufacturing cost becomes high.
In order to solve any such problems, a method should be adopted in which by having a component as a bonding agent co-exist with SAP in holding SAP so that the holding of SAP and the function of the bonding agent making a non-woven fabric are made to work at the same time. In general, a non-woven fabric is bulkiest when it is in an unbonded web and loses its bulkiness when it is finally made into a non-woven fabric.
The present invention makes it possible to manufacture a highly absorbent composite sheet having little SAP dropping out of it at both dry and wet states by a method in which an unbonded web in an original raw material condition is prepared, a liquid phase is formed in which a component is mixed to co-exist in the web to bond SAPs with each other and to function as a bonding agent to the web, the mixed liquid system is added to the web to stabilize it as a composite, and then liquid remaining in the web is removed, heat treated and then dried whereby the web is made into a non-woven fabric, the web is bonded with the SAP and the bonding of the SAPs is completed.
That is to say, the present invention relates to a method for manufacturing a highly absorbent composite sheet mainly consisting of a fibrous substrate web, a super absorbent polymer resin and a component to bond the resin and the substrate, wherein (a) said fibrous substrate web is a-yet-to-be-bonded web having little constituent fibers bonded with each other, (b) a liquid mixture system mainly consisting of a medium containing said highly absorbent polymer resin and said bonding agent, (c) a composite web is formed by adding said liquid mixture system to said fibrous substrate web, and (d) liquid component remaining in the composite web is removed whereby said highly absorbent polymer resin is made to be fixed to said fibrous substrate web and the webs comprising said fibrous substrate web are bonded with each other at the same time.
In the present invention, a preferable unbonded web may be a carded web or a laminated carded web and a carrier for guide may be used together with the carded web.
An unbonded web as obtained at dry state may be pretreated by means of a pretreatment liquid consisting of water or a medium miscible with water.
Also, an yet-to-be-bonded web may be an aqueous web to be obtained by a wet formation method or its laminate. Such unbonded web may be obtained by preliminarily treating a carded web or a wet formed web under a high pressure water stream.
The fiber component comprising an unbonded web is preferably a combination of easy-to-thermally-fuse fibers and synthetic fibers, and the fiber component is preferably finer than 2d composed of a first fiber layer mainly consisting of finer than 10d hydrophobic synthetic fibers and a second fiber layer mainly consisting of finer than 3d hydrophilic fibers.
A yet-to-be-bonded web may be formed from opened fibers of a wood pulp and easy-to-be-thermally-fuse fibers of 20 mm or shorter.
In the present invention, as examples of a liquid mixture system, such systems may be applied as a system where in a 1% or less melted solution of polyethylene oxide having a molecular weight of 100,000 or more particulate highly absorbent resin is dispersed to make a slurry, a system where in an aqueous emulsion of an ethylene-vinyl acetate copolymer a particulate highly absorbent resin is dispersed to make a slurry, a system where in an aqueous slurry of a highly absorbent resin containing a solvent system to be obtained by negative phase suspension polymerization and an aggregated gel of a highly absorbent resin to be obtained by aqueous solution polymerization are diluted in polypropylene glycol to make an easy-to-flow mixture, and a system where in an aqueous dispersion liquid of microfibrillated fibrils having a hydrating property a particulate highly absorbent resin is dispersed to make a slurry.
To this liquid mixture system, microfibrillated fibrils having a hydrating property may be added.
Furthermore, to said pretreatment liquid, microfibrillated fibrils having a hydrating property may be added.
In the present invention, a highly absorbent resin is preferably the one cross-linked on the surface so that it is 0.9% saline and has an AUL (absorbance under load) of 25 ml/g or higher under 20 g/cm2.
As other specific examples of a highly absorbent resin, an amino acid type polymer having a main component of aspartic acid and a polyacrylic acid type polymer with surface cross-linking omitted are listed.
Microfibrillated fibrils having a hydrating property are specifically microfibrillated cellulose or bacteria cellulose consisting of cellulose.
The micro fibrillated fibril cellulose fibers may be used as such systems as a system where the fibril cellulose fibers are uniformly dispersed at 1.5% to 0.2% concentration in a mixed medium of water and propylene glycol and particulate highly absorbent resin is dispersed at 5% to 50% concentration in the dispersion liquid to make a slurry, a system where the fibril cellulose fibers are uniformly dispersed at 1.5% to 0.2% concentration in a mixed medium of water and ethylene alcohol and particulate highly absorbent resin is dispersed at 5% to 50% concentration in the dispersion liquid to make a slurry, and a system where the fibril cellulose fibers are uniformly dispersed at 1.5% to 0.2% concentration in a three component mixed medium of water, ethanol and propylene glycol and particulate highly absorbent resin is dispersed at 5% to 50% concentration in the dispersion liquid to make a slurry.
A highly absorbent composite sheet according to the present invention is preferably a highly absorbent composite sheet consisting of an unbonded web (A), said highly absorbent resin (B) and said bonding component (C), wherein the percentage of said highly absorbent resin (B) is 50% or more satisfying the following formula:
B/(A+B+C)xc3x97100xe2x89xa750
More preferably, a highly absorbent composite sheet according to the present invention is a highly absorbent composite sheet consisting of an unbonded web (A), said highly absorbent resin (B) and said bonding component (C), wherein the percentage of said highly absorbent resin (B) in the total of said highly absorbent resin (B) and said bonding component except for said yet-to-be-bonded web which are absorbance contributing components is 70% or more satisfying the following formula:
B/(B+C)xc3x97100 xe2x89xa770